1. Field of the Invention
The present invention relates to a mobile communication system, a mobile station, a base station, and a control channel allocating method.
2. Description of the Related Art
An example of an ARQ (Automatic Repeat Request) feedback channel includes a feedback channel used for HSDPA (High Speed Downlink Packet Access) (for example, Non-Patent Reference 1).
Under HSDPA, a base station can simultaneously transmit data to multiple mobile stations for each transmission time unit (TTI: Transmission Time Interval) by applying CDM (Code Division Multiplexing) to an HS-PDSCH (High Speed Physical Downlink Shared Channel). The HS-PDSCH is a physical channel for transmitting traffic data. Accordingly, the multiple mobile stations simultaneously transmit ACK/NACK information (ARQ feedback signal) to the base station on HS-DPCCHs (High Speed Dedicated Physical Control Channels) as a result of decoding data transmitted on the HS-PDSCH.
An HS-DPCCH is a dedicated physical channel, whose signals are spread with a mobile-station-specific spreading code (scrambling code) in order to identify ARQ feedback signals transmitted from multiple mobile stations. Accordingly, the base station need not re-allocate a physical channel parameter such as a spreading code (scrambling code) for transmitting an ARQ feedback signal for each TTI, while the HS-DPCCH is allocated to a mobile station. The base station also need not transmit information about the allocated physical channel to the mobile station, while the HS-DPCCH is allocated to a mobile station.
FIG. 1 shows an example of the HS-DPCCH, which is a physical channel for transmitting ARQ feedback signals in HSDPA. On the HS-DPCCH, a set of control information is transmitted using three time slots. The HS-DPCCH is code-multiplexed with the other physical channels for transmitting uplink signals on data channels or the like. The signal sequence after code-multiplexing is spread with a user-specific spreading code (scrambling code) and then transmitted to the base station.    [Non-Patent Reference 1] 3GPP TS 25.308    [Non-Patent Reference 2] 3GPP TSG-RAN#26 RP-040461